Participants in this research study have a urea cycle defect or are likely to have a urea cycle defect because they have a child who is affected. Participants are likely to have undergone gene therapy or will undergo gene therapy at the University of Pennsylvania although that is not necessary for participation. The purpose of this study is to measure the rate at which the body converts ammonia into urea either to help decide if the participant is a carrier for a urea cycle defect because they have a child who is affected, or to monitor the participant to see if gene therapy is effective. All humans continuously produce ammonia, which they convert to urea. This study will measure how fast the participants' body converts ammonia into urea. The study involves taking by mouth (dissolved in water in a volume of 4 ounces, about half a glass) a special form of ammonia (nitrogen-l 5 ammonia) from which the body produces a special form of urea (nitrogen-15 urea). Nitrogen-15 ammonia is ammonia with a nitrogen atom that is heavier than normal. With an instrument called a mass spectrometer we can measure exactly how much nitrogen-15 urea is produced. Some people do not produce urea at a normal rate because they are born without a necessary enzyme ornithine transcarbamylase (OTC). These people tend to accumulate high concentrations of ammonia in their body. The test we are developing will let us discriminate people who carry the ornithine transcarbamylase (OTC) gene deficiency from people who do not have the deficiency. For example, by doing the test before and after carriers receive gene therapy for this disorder, we will know if such treatment improves the ability of these people to convert ammonia to urea.